More than 900 million light years away

It is the first time that gravitational waves have been observed from a combination of neutron stars and black holes.

Tnel of the Virgo gravitational wave detector, in Italian TuscanyALBERTO DI LOLLI

The catalog of gravitational waves is expanding. These ripples or disturbances in space-time first detected in 2015 are caused by violent phenomena in the universe. Until now, scientists working with the LIGO (in the US) and Virgo (in Italy) detectors had managed to detect gravitational waves from the merger of two black holes and two neutron stars respectively.

Now, for the first time, they have seen mixed gravitational waves, coming from a combination of neutron stars and black holes.

As detailed by the authors of the finding this Tuesday in the magazine Astrophysical Journal Letters, the two new gravitational waves, called GW200105 and GW200115, were detected in January 2020 and they were generated at a distance of more than 900 million light-years from our planet, after the capture of neutron stars by black holes.

The first of the mergers, GW200105, was captured by one of the two LIGO observatories (the one in Livingston, Louisiana) and the one in Virgo (in Italian Tuscany) on January 5, 2020. From the wave signal They found that it was generated at a distance of 900 million light-years from Earth and that it came from the merger of a black hole of 8.9 solar masses and a smaller compact object, of 1.9 solar masses, a star neutrons.

To the scientists’ surprise, 10 days later came the signal of the second gravitational wave (GW200115) to the two LIGO observatories (in Livingston and Hanford) and to Virgo. In this case, it came from the merger of a 5.7 solar mass black hole and a 1.5 solar mass neutron star at a distance of 1 billion light-years from Earth.

As Toni Font, professor of astronomy and astrophysics at the University of Valencia and leader of the group of gravitational waves at this center, explains to this newspaper, “in these two events, Black holes appear to have engulfed the corresponding neutron stars in one piece compaeras “.

The detection of any of the three types of gravitational waves captured so far, explains the Spanish scientist, is based on the same technique of filtering by recognition of patterns or waveforms obtained by solving the equations of General Relativity. “Perhaps for the mixed system we do not have as many patterns as for the case of two black holes, since to obtain them, the physics that describes the neutron star must also be incorporated into the mathematical model (intense magnetic fields, equations of state of dense matter, etc.) which makes it very expensive to solve the equations, “he details via email.

In the case of binary systems consisting of a neutron star and a black hole such as those that have been announced now, as is the case with the binaries of two neutron stars, “the gravitational signal can be accompanied by electromagnetic radiation, which can also help its detection and the spatial location of the source. For electromagnetic radiation to occur, the neutron star must be subjected to such intense tidal forces as it approaches the black hole, so that before being engulfed it undergoes a complete rupture, transforming into a disk of high-density matter around it. of the black hole “.

New goals

What happened next in the search for gravitational waves? “At the moment, the three possible combinations of compact binaries traditional that emit in the appropriate frequency range for the LIGO, Virgo and Japanese KAGRA detectors, have already been detected. There are other binaries waiting to be detected, those formed by two white dwarf stars, but their characteristic frequency is too low for terrestrial detectors, although they will be the target of the future LISA space detector “, says Toni Font.

In addition, he adds, “within black hole binaries, it would of course be very interesting to detect signals from objects of sub-solar mass, due to the cosmological implications that this could have. And also, of course, to detect continuous signals from plasars. or transient signals originated by massive stars when exploding as supernovae. In addition, there are many exotic possibilities that are the object of search, such as signals produced by cosmic strings or those produced by ultralight bosonic fields that could exist around black holes “, he advances.

Several Spanish teams from the University of Valencia (UV), the University of the Balearic Islands (UIB), the Institute of Theoretical Physics (IFT) of the Autonomous University of Madrid-CSIC have a prominent role in the LIGO-Virgo collaboration. the Galician Institute of High Energy Physics (IGFAE) of the University of Santiago de Compostela (USC), the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of High Energy Physics (IFAE) of Barcelona.

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